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24                                 CHEMICAL ENGINEERING
There are three methods of compounding, by which this characteristic is modified:
1.  The steam expands in several successive pressure stages'(nozzles), a moderate reduction of pressure and generation of velocity occurring at each nozzle.    The number of rows of buckets equals the number of pressure stages.    Each row receives steam at moderate velocity and may itself run at moderate speed, discharging its steam to the next set of nozzles.    The De Laval multi-stage machine, of this type, uses peripheral speeds not exceeding 650 ft. per second, with good efficiency.    The Kerr turbine has 2 to 13 stages with peripheral speeds from 300 to 600 ft. per second.
2.  The steam is allowed to leave the buckets at high velocity, but is immediately deflected and caused to strike either another row of buckets or the same row of buckets at a new point.    Several such traverses may be provided.    The Terry and some of the smaller Westinghouse turbines are of this type, both using a single row of buckets.    The Curtis uses several rows of buckets.    Velocity compounding, as thus described, is probably less efficient than the use of successive pressure stages, but leads to compact and sometimes inexpensive construction.    The peripheral speeds may be somewhat reduced from those of simple machines without impairing the efficiency.
3.  A combination of methods (1) and (2) may be used, as in the Curtis machine which may have, say, five pressure stages with two moving wheels in each pressure stage.    This leads to peripheral speeds as low as those of pressure turbines, along with high efficiency.
Size and Capacity.—The wheel diameter of an impulse turbine is determined by speeds rather than by capacity, the buckets not being filled with steam.
The nozzle dimensions (first stage nozzles in a compound machine) determine the steam flow, and thus (type and proportions being fixed) the output. Whenever the outlet pressure is less than 0.58 the inlet pressure the nozzle will have a diverging outlet. The weight of steam it will discharge is, however, determined by the throat (most contracted) area, the outlet area determining only the outlet pressure. The weight of saturated steam in pounds per hour that will flow through a throat area of a sq. in., is 60 ap°-97 •*• ic, where p and x are the absolute pressure and dryness fraction of the inlet steam, respectively. For steam superheated T°, the weight is less, being equal to 60 ap«-*7 -r- (1 + 0.0006577).
For pressure turbines, the area of the annulus at the low-pressure end is an index to the steam flow. About two-thirds the area may be effective, the remainder being used up by the bucket thickness. The velocity to be considered is that normal to the disc, or the steam velocity multiplied by the sine of the exit angle of the buckets: say by sin 20°. The specific volume is that of wet steam at the exhaust pressure, say about 275 cu. ft. at 28 in. vacuum. Then at a maximum steam velocity of 900 ft. per second, if the diameter of the (single) low-pressure drum is d in. and the blade height (last row) is h in., the weight of steam discharged per hour is 58.8ft (d + ft) Ib. Thus for d = 25, ft = 5, the weight of steam is 8,820 Ib. per hour.
Overload capacities of impulse turbines are very high. Additional nozzles may be used, or in compound machines steam at boiler pressure may be admitted to the secondary nozzles.
In addition to the impulse utilized on the forward edge of the buckets, most commercial pressure turbines are built in combination with an impulse wheel. The objectionable features of superheat, if used, are then confined to stationary nozzles: provision for overloads is readily made: and end thrust may be eliminated by causing the steam to flow both ways from a central impulse wheel through two sets of pressure elements (double flow turbine).
Turbine Economy.—The ideal steam rates, pounds per kilowatt-hour, may be summarized as follows: upper figures are for non-condensing machines at 16 Ib.